Following an operation involving the destruction of tumorous tissue by freezing with a cooled probe inserted into the body, it is necessary to thaw or allow to be thawed the tissue immediately surrounding the probe in order that the latter may be withdrawn. Such thawing is commonly carried out by one of two principal methods. The first involves the use of latent body heat. This has the serious disadvantage that the process is extremely slow, thereby significantly increasing operation times. The second method utilises electrical heating of the probe tip. Although this is a considerably faster method, there are inherent safety problems which necessitate the utmost care in producing and using equipment of this kind.